1. Field of the Invention
The present invention relates to optical modulators used in optical communication systems for modulating data signals from electrical signals into optical signals by controlling optical phases by means of an electro-optic effect.
2. Description of the Related Art
Recently, optical communication systems have progressed in various fields with the aim of faster communication, larger capacity, and size reduction. For optical transmitting devices, external modulators of the optical waveguide type using electro-optic crystals such as lithium niobate (LiNbO3; hereinafter referred to as LN) substrates have been developed in view of wideband characteristics and chirp characteristics instead of direct modulation of laser diodes.
An example of such modulators is that including a Mach-Zehnder optical waveguide formed on an electro-optic crystal substrate composed of, for example, LN by thermal diffusion of a metal film composed of titanium (Ti) or the like partly formed on the LN crystal substrate or by proton exchange in a benzoic acid after the formation of a metal film. A buffer layer composed of silicon dioxide (SiO2) or the like is formed on the substrate having the optical waveguide formed thereon. Furthermore, in a case of a Z-cut substrate where the LN material is cut along a direction parallel to the Z axis of the LN crystallographic axes, a signal electrode connected to a radio-frequency (RF) signal generating source that generates modulating signals is formed above one of a pair of linear portions of the Mach-Zehnder optical waveguide, and an earth electrode to which a ground potential is applied is formed above the other linear portion. In general, in the case of coplanar electrodes, the electrode structure is symmetrical about the signal electrode, and two earth electrodes are formed so as to have the signal electrode disposed between the earth electrodes. When signals are applied between the signal electrode and the earth electrodes, electric fields are generated in the optical waveguide, and the refractive index of the optical waveguide is changed. With this, the phase difference between light beams propagated through the pair of linear portions of the optical waveguide is changed. For example, when the phase difference is zero, the optical output is determined as logical level 1, whereas the optical output is determined as logical level 0 when the phase difference is π.
In order to realize such modulation, a direct current can be superposed on the high-speed RF signals using a bias T such that an operating point of the RF signals (DC bias component) applied to the signal electrode is controlled. However, this leads to an increase in operational costs or the like. Moreover, it is difficult to apply the bias T to an optical modulator driven with signals of 10 GHz or higher due to limitation of the frequency band caused by the bias T. In order to solve this problem, an optical modulator disclosed in Japanese Unexamined Patent Application Publication No. 2003-233042 includes a bias electrode for applying a bias voltage so as to control an operating point of RF signals (DC bias component) applied to signal electrodes. The signal electrodes are disposed above parts of linear portions of the Mach-Zehnder optical waveguide, and the bias electrode is disposed above other parts of the linear portions.
However, according to the above-described structure, the chip length of the optical modulator becomes long since the bias electrode is disposed downstream of the signal electrodes in a direction along which light beams travel in the optical waveguide. In order to solve this problem, an optical modulator having a bias electrode disposed adjacent and parallel to a signal electrode is disclosed in Japanese Unexamined Patent Application Publication No. 10-54961. However, according to the structure of this optical modulator, the bias electrode may be shorted with the earth electrode.